During the conversion of peat to coal, methane gas is produced as a result of thermal and biogenic processes. Because of the mutual attraction between the coal surface and the methane molecules, a large amount of methane can remain trapped in-situ as gas adhered to the organic matter (i.e., carbonaceous materials) in the formation. The reserves of such "methane" in the United States and around the world are huge. Most of the reserves are found in coal, but significant reserves are found in gas shales and other solid carbonaceous subterranean formations.
Conventional methane recovery methods are based on reservoir pressure depletion strategy; that is, methane is desorbed from the carbonaceous surfaces by reducing the reservoir pressure. While this method of methane production is simple, it is not efficient. Loss of reservoir pressure deprives the pressure depletion process of the driving force necessary to flow methane gas to the wellbores. Consequently, the gas production rate from a well is adversely affected by the reduction in reservoir pressure.
Another method of recovering methane is by injecting into the solid carbonaceous subterranean formation a gas, such as CO.sub.2, having a higher affinity for coal or other carbonaceous material than the adsorbed methane, thereby establishing a competitive adsorption/desorption process. In this process, the CO.sub.2 displaces methane from the surface of coal, thereby freeing the methane so that it can flow to a wellbore and be recovered. This method is disclosed in the reference by A. A. Reznik, P. K. Singh, and W. L. Foley, "An Analysis of the Effect of CO.sub.2 Injection on the Recovery of In-Situ Methane from Bituminous Coal: An Experimental Simulation," Society of Petroleum Engineers Journal, October 1984. The problem with this method is the large volume of CO.sub.2 that must be injected into the solid carbonaceous subterranean formation in order to exchange sites with methane. In most instances, such an amount would be uneconomical. This reference reports that mixing even small amounts of nitrogen gas with CO.sub.2 significantly reduces the effectiveness of displacement desorption of methane by CO.sub.2.
There is a need for a method of producing methane from coal and other solid carbonaceous subterranean formations that accelerates the production rate and improves recoverable gas reserves economically.